This invention concerns a joint between adjacent pieces of fiber reinforced composite pipe which is suitable for use with conventional fiberglass pipe, high strength carbon fiber composite pipe or with high strength fiber reinforced pipe containing embedded steel strips.
Fiber reinforced composite pipe finds appreciable utility where corrosive materials are carried in a pipeline or where the pipeline is buried or laid on the sea floor or is otherwise subjected to an external corrosive environment. Techniques have been developed for producing fiber reinforced pipe for carrying high internal pressures. For example, until recently a typical high pressure pipe might have a 10 cm nominal diameter and an internal burst pressure of about 600 bar. More recently, fiber reinforced high pressure pipes with a 20 cm nominal diameter have been rated at about 1200 bar burst pressure.
Such fiber reinforced composite pipe, when reinforced with glass fibers, may have a wall thickness on the order of 5 cm, which clearly makes it costly and heavy. There is currently development of another variety of high pressure pipe which includes helically wound steel strips embedded in fiber reinforced resin. Such an embodiment has such good strength that the wall thickness may be as little as 7 mm for a 25 cm nominal diameter piper. Such a pipe is described and illustrated in U.S. Pat. No. 4,351,364, for example.
A substantial concern in such high strength pipe, either fiber wound or with steel reinforcement, is the coupling or joint between adjacent pipes. The pipe joint needs to have a circumferential burst strength at least as great as, and preferably more than, the principal length of pipe. More significantly, the joint must have sufficient longitudinal shear strength to prevent the pipes from separating under internal pressure or other axial loads. Preferably the joints are designed to have sufficient longitudinal shear strength that they will not fail before rupture of the pipe itself.
Design of a suitable joint for fiber reinforced composite pipe differs appreciably from metal since the fiber reinforced composite pipe, as contrasted with steel, for example, has very little ductility. This places significant limitations on what can be done in pipe joints. In a conventional bell and spigot joint secured by filling the joint with adhesive, the high stiffness of the adherent places a high shear stress on adhesive in the joint. The distribution of stress along the joint is not uniform. The shear stress is quite high at the ends of the adhesive, as much as three times the average stress, and decreases rapidly from the ends toward the middle. In a long adhesive joint, the shear stress in the middle of the joint may be near zero.
The high stress at the ends of adhesive in such a lap shear joint can result in failure of the adhesive in shear adjacent to an end of the joint. This simply transfers the shear stress further along the joint and there is progressive failure at average stresses that would appear to be well within the capability of the adhesive.
Other joints for fiber reinforced pipe are also difficult because of the stiffness of the fiber reinforced composite. It is desirable to provide a pipe joint that redistributes stress along the length of the joint to avoid such progressive failure of the joint. Preferably the joint has a higher strength than the wall of the pipe remote from the joint. The pipe joint should have a high safety margin, i.e. a failure stress greater than the rated capability of the joint. The joint should be easily and economically assembled in field conditions.